Conventional diesel engines produce less gaseous hydrocarbon (HC) and carbon monoxide (CO) than gasoline engines and it is possible to meet present legislated limits for these components using a platinum (Pt)-based diesel oxidation catalyst (DOC). Diesel nitrogen oxides (NOx) emissions are presently controlled by engine management, such as exhaust gas recirculation (EGR). As a consequence, however, diesel particulate matter (PM) emissions including volatile and soluble organic fractions (VOF and SOF respectively) are increased. The DOC is used to treat VOF and SOF in order to meet presently legislated limits for PM.
Two ways of reducing compression ignition engine emissions, which can be used in addition to exhaust gas after treatment, are engine management and engine design. More recently, a new generation of compression ignition engines have been developed which use a range of engine management techniques to lower the combustion temperature. One such technique is for substantially all fuel for combustion to be injected into a combustion chamber prior to the start of combustion.
An advantage of these techniques is that they can reduce NOx and PM emissions, without significantly increasing fuel consumption. An embodiment of the new generation of engines which employs these techniques is known as a Homogeneous Charge Compression Ignition (HCCI) diesel engine. Characteristics of an HCCI diesel engine include homogeneous fuel charge for external or internal mixture formation by variable valve timing, increased swirl ratio, injection rate control (multiple injection) and adapted spray configuration; high dilution rate for a moderate burn rate; low NOx by charge dilution and low combustion temperature; and low PM by prolonging the time for mixture preparation and, consequently, homogenisation. All relative terms are compared to a normal direct injection diesel engine.
Another new compression ignition engine is known as the Dilution Controlled Combustion System (DCCS), for example Toyota's Smoke-less Rich Combustion concept. Characteristics of DCCS include conventional direct injection; extremely high dilution rate to lower combustion temperature below soot formation threshold by increasing ignition lag, increase in swirl ratio, variable valve timing and injection rate control (multiple injection); low NOx and PM by very high charge dilution rate and extremely low combustion temperature; and very high EGR rate. All relative terms are compared to a normal direct injection diesel engine.
By contrast, a typical direct injection light-duty diesel engine produces approximately 50 ppm NOx, 1000 ppm CO and 800 ppm HC (C1) at idle (exhaust gas temperature about 185° C.); and approximately 1250 ppm NOx, 70 ppm CO and 30 ppm HC (C1) at high load (exhaust gas temperature about 500° C. (all values engine out)).
We have investigated the emissions of a vehicle including one of the new generation of engines, and have found that, despite the improvements in reduced NOx and PM, they can produce high levels of CO relative to a conventional direct injection diesel engine. Such CO emissions can be characterised by an exhaust gas composition of >2000 ppm CO, such as >2500-10000 ppm CO e.g. >3000 ppm CO, >4000 ppm CO, >5000 ppm CO, >6000 ppm CO, >7000 ppm CO, >8000 ppm CO or >9000 ppm CO, below e.g. about 250° C. during conditions wherein substantially all fuel for combustion is injected into a combustion chamber prior to the start of combustion.
Additionally, we have observed that such diesel engines can produce a relatively high level of HC below e.g. about 250° C., e.g. less than 200° C. or less than 150° C., during low NOx operating conditions, such as >500 ppm e.g. from 600-1000 ppm, illustratively 700 ppm HC, 800 ppm HC or 900 ppm HC, C1 unburned hydrocarbon (HC).
Furthermore, we believe that unsaturated hydrocarbons can result from the incomplete combustion of diesel fuel, examples of which are ethylene, propylene, aromatics and polyaromatics. Release of certain unsaturated HCs is undesirable for environmental and health reasons.
It is known that current direct injection diesel engines can produce exhaust gas comprising >2000 ppm CO under certain operating conditions, e.g. at cold start as part of a warm-up strategy or following hard acceleration. However, we believe that the current diesel engines do not emit such high levels of CO under normal driving conditions or such high levels of CO in combination with such high levels of HC during normal operation, e.g. at temperatures of up to 250° C.
In our EP 0341832 we disclose a process for combusting diesel particulate deposited on a filter in nitrogen dioxide (NO2) at up to 400° C., which NO2 is obtained by oxidising nitrogen monoxide (NO) in the exhaust gas over a suitable catalyst disposed upstream of the filter. The NO oxidation catalyst can comprise a platinum group metal (PGM) such as Pt, palladium (Pd), ruthenium (Ru), rhodium (Rh) or combinations thereof, particularly Pt. The filter can be coated with material which facilitates higher temperature combustion such as a base metal catalyst, e.g. vanadium oxide, La/Cs/V2O5 or a precious metal catalyst. Such a system is marketed by Johnson Matthey as the CRT®.